Field of the Invention
This invention relates to additives for electroplating baths which enhance the quality of electroplated products and improve the efficiency and safety of the electroplating process. In particular, the invention relates to additives which suppress the build-up of foam in the electroplating bath during mass production, and to additives which control the thickness and content of the deposit over a wide range of current densities, and to additives which act as brighteners.
This invention also relates to additives for electroplating baths which prevent the oxidation of divalent tin in the baths, but yet not adversely affect the grain boundaries of the plated metal or alloy.
The additives are particularly useful, for instance, in alloy systems such as a tin/lead electroplating bath.
Automatic plating lines for printed circuit boards and connectors have introduced new requirements into the properties of industrial electroplating baths. This is especially true, for instance, for solutions used in producing alloy deposits such as a tin/lead alloy in a continuous on-line mode. A typical apparatus for conducting such electroplating consists of a reservoir holding the plating solution, a tank where the electroplating is done, and a pump which propels the plating solution from the reservoir to the plating tank. The plating solution is usually returned to the reservoir to complete a circuit. Work pieces may be moved through the plating tank on a conveyor belt at a regulated speed and at a current density commensurate with the acceptable alloy deposit.
For economic reasons, the automatic line carrying the work pieces should preferably move at a high speed and at elevated current densities. When work pieces are moving through the system at such a high rate, the plating solution should preferably be pumped rapidly through the system as well. Such high speed pumping agitates the solution and, due to the prior art additives used in the bath, may generate undesirably high quantities of foam. For instance, commercial tin/lead electroplating solutions formulated in recent years usually contain nonionic surfactants based on polyethylene oxide, and/or poly(ethylene/propylene) oxide copolymers. These surface active agents act as grain refiners and wetting agents. However, solutions containing such nonionic surfactants may build up stable foams in undesirable quantities when subjected to the aeration and agitation that is common in most high-speed commercial mass production systems.
In an automatic plating line, the usual pumping action so agitates the plating solution that foam is generated to an extent capable of overflowing the holding tank. This may result in an economically detrimental loss of plating solution. Such foaming and loss of plating solution may be reduced by slowing down the plating process, but at the expense of lost productivity. Prior art attempts to solve the foaming problem without reducing speed have included addition of anti-foaming agents which, while effective in reducing foam, may undesirably introduce impurities into the deposit.
During electroplating, the work pieces may have variations in current density along their platable surfaces due to variations in surface geometries and the like. This may cause variations in the depth of the deposited plate, or in some instances involving alloys, variations in the ratios of metals deposited in the plate. For instance, in tin/lead electroplating, higher lead content may be observed in areas having low current density relative to areas having higher current density. Many articles, such as printed circuit boards, go through a fusion or reflow process after alloy plating. During this process, dissimilarities in alloy composition may produce an undesirable roughness in the fused coatings. Also, areas of high current density during electroplating will tend to have deposit thicknesses greater than those of low density areas. These differences in thicknesses are inappropriate in many contexts.
U.S. Pat. No. 4,000,047 discloses a bath for electrodeposition of bright tin/lead alloys having, among other things, certain specified pyridines and quinolines. U.S. Pat. No. 4,388,161 discloses preparation of bright tin/lead alloy deposits by electrocoating using a bath having, among other things, certain halomethyl substituted dioxolanes which are further substituted by a specified phenyl, naphthyl or pyridyl radical.
In U.S. Pat. No. 4,555,314, long chain alkyl guanamines are disclosed for regulating tin/lead ratios during electrodeposition of tin/lead alloys. U.S. Pat. No. 4,565,609 describes the use of high molecular weight quaternized 2-alkyl imidazolinium salts for similar purposes. U.S. Pat. No. 4,459,185 discloses the use of high molecular weight cationic compositions in conjunction with amphoteric surfactants as regulators. There remains a need, however, for more effective regulators for maintaining more consistent alloy content and coating thickness along the surface of the platable materials. A difficulty in the prior art has been finding additives to regulate both thickness of alloy deposit and ratio of metal concentrations in alloy at low additive concentrations such that the additives do not contribute significantly to occluded carbon content of the deposit.
In some products, electrodeposited alloy may be subjected to a high temperature flow or fusion process capable of converting solder plate into a smooth bright coating. However, use of such a fusion process may be inappropriate for many electrical and electronic components including those electroplated with various alloys. To produce a bright electrodeposit on these products, selected additives must be formulated into the bath. Prior art additives for these purposes include numerous aldehydes such as formaldehyde, acetaldehyde, 3-hydroxy butyraldehyde, and chloro-substituted aromatic aldehydes. Although blends of these compounds are effective in producing a bright finish, they are also quite volatile and, under conditions of the plating operation, concentrations sufficient to constitute a health hazard may accumulate in the immediate environment.
In addition to the foregoing, the ready oxidation of divalent tin to the tetravalent state by atmospheric oxygen is well recognized in those applications where stannous tin is electrodeposited on a substrate in a variety of technical applications. The formation of metastannic acid and its derived products by the aerobic and anodic oxidation of stannous tin introduces a serious problem in the functioning of tin and tin alloy electroplating baths. This is manifested by the formation of colloidal and opalescent solutions which lead to the formation of sludges that have to be removed by continuous filtration. The maintenance of filtering systems and the disposal of appreciable quantities of sludge containing toxic metal residues represents a serious industrial problem.
It is well known in the prior art that the oxidation of divalent tin in electroplating baths can be inhibited and substantially reduced by the use of antioxidants. Among those commonly used are the dihydric phenols, i.e., hydroquinone, catechol, and resorcinol. In U.S. Pat. No. 4,871,429 resorcinol and certain other hydroxyphenol compounds are used to suppress formation of tetravalent tin. Other antioxidants that have been mentioned for this use are the substituted 1,4-phenylene diamines and the 4-(N-alkylated) aminophenols. Although all of the above compounds function as acceptable antioxidants for these baths, they may have adverse effects on the physical characteristics of the deposited coating.
In the fabrication of printed circuit boards, connectors, and other electronic devices, a matte or semi-bright deposit of a tin lead alloy is required. The end use requirements of the devices frequently dictates the physical properties of the tin or tin lead alloys electrodeposited on the work pieces. For the most part, the nature and shape of the metallic grains deposited on the surface and its boundaries determine the physical properties of the composite surface. These in turn determine the functional applicability of the electrodeposited coatings for their end use.
Although the polyhydric phenols, mentioned above, act as antioxidants for stannous tin, they have undesirable properties of adversely affecting the grain boundaries of the plated metal or alloy. This effect can be studied by scanning electron microscopy. Under these magnifications, the deposited metal is composed of relatively large granules having an irregular surface of many high and low areas.